JPH05507609A - Overvoltage/overcurrent protection circuit with excellent ground balance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] The present invention connects to a two-wire line that transmits analog and digital signals in both directions. This is related to overvoltage and overcurrent protection circuits for replacement terminals that are now being used. . Terminal circuits can convert two-wire connections to four-wire connections, five-wire connections, or higher-order connections. It is something that can be converted into a connection format, and mainly depends on the impedance of the transmitting/receiving device. It works to match the impedance of the line with the impedance of the line. Protection circuit is l5DN circuit network In this case, the so-called network end form the end. The overvoltage and overcurrent protection circuit is the R interface of the l5DN circuit network. , also implemented in the switching terminal circuit provided at the S interface and T interface. can do. The protection circuit according to the invention can be used on the central office side or on the subscriber side of a conventional telephone network. It can also be provided in the exchange terminal circuit on the user's side.

Background technology A network terminal capable of converting from a two-wire connection to a four-wire connection is provided by the applicant. It is disclosed in US patent specification US-A-4°539.443.

Switched terminal circuits are usually located within the line terminals of telephone exchanges and connect to two-wire lines. It has two input terminals for this purpose. Terminal circuit must fully meet many requirements must be able to do so. For example, the input terminals of a circuit are connected to a reference potential, usually ground. There must be a balance. Otherwise, so-called relay currents will occur on the line. Because it's cheating. Additionally, the exchange terminal circuit must exhibit high return loss. . Within the general frequency band, i.e. the audio frequency band or the digital signal In the cut frequency band, the effective attenuation distortion and fundamental attenuation must be small.

Another requirement is that between transmitters and receivers on the subscriber side or telephone exchange side, etc. This means that the so-called loop attenuation must be small.

In the case of a network terminal, the terminal cannot be connected to the power supply through the input terminals of the circuit as well. Must be. This power supply provides current to the subscriber equipment at the other end of the line. child electric current is part of telephone methods and is commonly used for the purpose of transmitting signals on railway lines. .

Known subscriber network terminals have a frequency range of about 300 Hz to about 3.5 kl (z). Conceived to transmit audio signals and, as is known, direct current signals is considered to be sent. However, if the subscriber line is lower than the voice frequency Problems may arise when used to transmit digital signals sent at much higher frequencies. Ru. Digital signals have much greater attenuation than audio signals, so High power levels are required to send the signal. For this reason, a better ground plane Balance will be required.

The purpose of the overvoltage/overcurrent protection circuit is to protect the terminal circuit or The purpose is to protect expensive equipment connected to the output side of the network terminal. Overvoltage For example, current is generated when lightning voltage is applied to the subscriber line and interferes with the IE magnetic field. For example, if an AC 220V power cable comes into unexpected contact with the railway line. etc. This can happen, for example, due to fire or carelessness. Or the railroad tracks and power cables are installed inappropriately close to each other, causing problems such as rough weather or animals. This is because the two came into electrical contact due to the influence of

The overvoltage and overcurrent protection circuit must not affect the electrical characteristics of the aforementioned terminal circuit.

Therefore, the equipment will not be damaged when the line is grounded or shorted. It is necessary to limit the current flowing through the device so that it does not occur. connected to the line The operating voltage of digital exchanges is usually 5V, so the maximum voltage is 5V. It is designed so that it will not be damaged. For example, the overvoltage that snow generates on the railway is 1500 Can also be used as a bolt.

Today's systems use large glow discharge tubes, i.e. cold shades, as the main means of protection. A pole tube is connected between each line of the subscriber line and earth at the input side of the telephone exchange. Ru.

These discharge tubes are slow in operation, with a response time of about 1 ms. Maximum electric power of lightning pulse Pressure appears after approximately 107170 seconds. Furthermore, the moment when the glow discharge tube is triggered is Because of the difference between the lines, a residual voltage is created across the two lines of the line.

Fire must never occur within the overvoltage/overcurrent protection circuit. As shown in Figure 1, In the case of the known overvoltage and overcurrent protection circuit, overvoltage is limited by two wire-wound resistors. It will be done.

If the overvoltage is of high voltage for a long period of time, these resistors will turn bright red. There is a risk of catching fire. These wire-wound resistors have an overvoltage protection circuit in the terminal circuit. matched to each other, i.e. in pairs, so as not to affect the earth balance of I have to be there. In order to be able to withstand high overvoltages, the resistor must All parts must have high durability against the nominal voltage, that is, voltage. Furthermore , the resistor must be physically large to withstand high power.

Traditional protection circuits also include line transformers. If the current flowing through the transformer is large , the steel foil of the circuit board on which the overvoltage/overcurrent protection circuit is mounted may start to burn. be.

It is known that a temperature-dependent resistor, a so-called PTC resistor, can be used as a current-limiting protector. It is being However, the disadvantage of these resistors is that the nominal voltage of the component is limited Therefore, when high voltage is applied to a component, electrical sparks are likely to occur within the component. That is true. Care must also be taken in the placement of these PTC resistors. too If the current flowing through the PTC resistor is large, the resistor will start to melt and the molten material will melt. can easily cause the underlying steel foil board or card to catch fire. PTC resistor under high current and high voltage If the resistor is exposed to a high temperature, a temperature gradient will occur within the resistor matrix, making it susceptible to cracking.

Then, the protective function no longer exists.

It is also known to use Zener diodes as a means of protection against overvoltage.

This type of Zener diode must have the steepest characteristics. . However, this poses a dynamic problem. That is, S many harmonics (overtones), Harmonic distortion and cross-modulation distortion occur, which adversely affect the quality of signal transmission. It is rustic.

The requirement for earth balance is particularly important, and abnormal voltages within the protection circuit can exceed up to 60 deg. It can reach as high as Shibel.

If the line impedance is approximately 600 ohms, this means that the line voltage protection circuit The error between the resistance values in the base symmetric circuit can only be tolerated by 0.1 ohm at most. It means something. Therefore, the operation of the railway line becomes difficult.

Summary of the invention An object of the present invention is to provide an overvoltage/overcurrent protection circuit with excellent ground balance. This allows the bracket protection circuit to be constructed from small and inexpensive components, making it easy to replace the It can be effectively used in a terminal circuit or network terminal.

Another object of the invention is a protection circuit of the type already mentioned, in which the primary winding of the transformer is Connected to the input terminal of the subscriber line, with a wire-wound current limiting resistor connected in series in the middle. It's about providing something that doesn't need to be done.

More specifically, this transformer forms part of the terminal circuit, i.e. This is the line transformer of the terminal circuit.

When an overvoltage pulse generated on the line is transmitted from the line to the equipment to be protected, In order to shorten the period due to the transformer effect, the line transformer in the terminal circuit is It should be actively used. The pulse energy transferred to the secondary side of the transformer is Because of this attenuation due to the generator effect, on the secondary side of the overvoltage/overcurrent protection circuit, It becomes possible to use small parts.

Return loss, effective attenuation distortion, basic attenuation, and loop attenuation are It can be significantly reduced within both frequency bands used for data bit transmission. Wear.

The mutual arrangement of parts is not strict. Zener diode included in the protection RIM path It is not necessary for either of them to exhibit steep characteristics.

Overvoltages and overcurrents are reduced step by step in different components within the protection circuit, so The nominal voltage of the product is lower, allowing the use of cheaper standard components.

A feature of the present invention is that one varistor and two PTC resistors are connected to the primary of the line transformer. It is used in earth balance circuits together with windings.

Brief description of the drawing The present invention will be described in detail below with reference to the drawings. Figure 1 shows a conventional overvoltage/overcurrent protection circuit. shows. FIG. 2 shows an overvoltage and overcurrent protection circuit according to the present invention. Figure 3 is shown in Figure 2. This figure shows the change in output voltage when a lightning pulse is applied to the overvoltage/overcurrent circuit shown.

Best embodiment of the invention FIG. 1 shows a conventional overvoltage/overcurrent protection circuit. This circuit is a known terminal circuit or is mounted on the network terminal and connected to the two lines of the subscriber line at input terminals a and b of the terminal circuit. has been done. In this case, the line terminal circuit has four output terminals TI.

T2. T3. It has T4. A transmitting device (not shown) is connected between terminals T1 and T2 A receiving device (not shown) is connected between terminals T3 and T4. In Figure 2 is the original.

Only important parts are shown. In order to function as a terminal circuit, the remaining These parts are required, but are not shown.

The primary winding of the line transformer LTI has two winding parts Ll.

L2, with a center point capacitor C connected in series between them. −Next winding A current limiting resistor Ra is connected between one end and the input terminal a, and a current limiting resistor Ra is connected between the other end and the input terminal A. A second current limiting resistor Rb is connected in series. Voltage source E is connected to resistor R1 It is connected across the center point capacitor C via R2. The cathode of voltage source E is It is grounded. Zener diode Zl, Z2. Z3. Group where Z4 is -48V between the quasi-voltage source and ground, and between the ends of the series combination of L1-C-L2, They are connected as shown. Current source E is used to supply direct current to subscriber equipment. I can stay. - The downstream circuit is symmetrical with respect to earth. Current limiting resistor Ra, R b consists of a large wire-wound resistor. Zener diodes 21 to Z4 actuate overvoltage. Acts as an overvoltage protector that shorts to the ground. Transformer LTI has a secondary winding L3 , L3 are connected in parallel with a capacitor CI. Zener diode Z5~Z 8 acts as an overvoltage protector for the transmitting/receiving device.

Although the circuit shown in Figure 1 has the drawbacks mentioned above, these drawbacks are not inherently This is due to the presence of resistors Ra and Rb.

FIG. 2 shows an overvoltage and overcurrent protection circuit according to the present invention. The circuit shown in Figure 2 Components corresponding to those in the circuit of FIG. 1 are given the same reference numerals. of this circuit The primary winding of line transformer LT2 consists of two winding parts Ll, L2, and the secondary winding is Two winding parts L3. It consists of L4.

The circuit on the primary side has series combinations Ll-C-L2 connected in series as in Fig. 1. . However, the important difference is that both ends of this series combination are directly connected to input terminals a and b. Note that the current limiting resistors Ra and Rb, which were connected in series in Figure 1, are not present. That is. Instead, the center point can be connected via PTC resistors PTCI, PTC2. The current is limited by a varistor Vl connected in parallel with the capacitor C. one One PTC resistor is connected between one end of the varistor and one end of the center point capacitor. , the other PTC resistor is connected between the other end of varistor V1 and the other end of the center point capacitor. It is connected. Resistors Ral and Ra2 act as current limiters, which will be discussed in detail later. As will be explained, these resistors are similar to the current limiting resistors Ra and Rb shown in FIG. In comparison, both volume and resistance are much smaller. The resistor Ral is connected to one end of the varistor and the voltage The resistor Ra2 is connected in series with one pole of the source E, and the resistor Ra2 is connected to the other end of the varistor and the voltage It is connected in series between the other pole of source E, in this case the cathode. This cathode is is being sourced.

The varistor is a metal oxide type and has symmetrical voltage/current characteristics. It is a permanent resistor. When the voltage exceeds the rated voltage, that is, the nominal voltage, the resistance value suddenly decreases. decreases. The response time of this type of varistor is less than 25 nanoseconds.

PTC resistors are also called cold conductors, and are temperature-dependent resistors whose resistance increases as the temperature rises. It is a semiconductor resistor. The resistance value suddenly increases when it reaches a predetermined temperature called the reference temperature. It becomes. Since the positive temperature coefficient is very large, PTC (positive temperature) perature coefficient) resistor.

The primary circuit operates as follows. If a lightning pulse is applied to the subscriber line, Capacitor C starts charging, so the voltage across the capacitor increases. of capacitor When the voltage exceeds the varistor's nominal voltage, the varistor Vl begins to conduct. The current flowing through the varistor increases rapidly. As a result – the current flowing through the next winding increases rapidly. Eventually, the transformer core saturates. After that, anything more than this is actually energy or transmutation. It is no longer transmitted to the secondary side of the device. Since the current flowing through the varistor increases, the resistor R The current that must be absorbed by al and Ra2 is much smaller than in the circuit of FIG. death The volume and resistance of resistors Ral and Ra2 are much larger than those in the circuit shown in Figure 1. It can be made smaller.

When the varistor Vl starts conducting, the current flows through the temperature dependent resistors PTCl and PTC2. It also flows. As a result, these temperatures rise and eventually reach the reference temperature, causing the resistor to The number of resistors increases rapidly. Therefore, the switching time of resistors PTC1 and PTC2 is becomes shorter.

As mentioned above, when the varistor becomes open, the current flowing through the negative winding accelerates. To increase. The current continues to increase until the core of transformer LT2 saturates. core is saturated After that, no more energy is transferred to the secondary side of the transformer. Until the moment of saturation The voltage supplied to the secondary side is reduced in two stages. Some parts are parallel to the secondary winding of the transformer. By the varistor v2 connected to the column, some are connected to the terminals TI, T2 Zener diode Z5. With Z6, one more The part is terminal T3. Zener diode to protect the receiver connected to T4 Code Z7. Z8 does that. For example, if the nominal voltage of varistor V2 is 6 If it is 0V, all voltages higher than 60V will be absorbed by varistor v2. did Therefore, Zener diodes Z5 to Z8 only absorb voltages lower than 60V. good. In the example of Figure 1, the Zener diodes Z5-Z8 can also handle voltages higher than 6ov. Since it is necessary to absorb all of the energy, Zener diodes have excellent characteristics. It is required that Particularly low current areas, i.e. Zener diodes are non-conducting This is so in the transition region from the state to the conducting state. Overvoltage protection circuit of the present invention In this case, there is little need for the dynamic slope of the resistance of the Zener diode to be steep.

Figure 3 shows a 1500V lightning pulse applied to primary side terminals a and b for 800 microseconds. Zener diode Z5. This shows the voltage across Z6. . In the darkness of IO microseconds, the snow pulse rises from 0 volts to I 500 volts. No. In Figure 3, the Y-axis represents voltage in volts and the X-axis represents time in microseconds. thunder pa The pulse begins at time t;0. When the voltage applied to the varistor Vl reaches 90 volts , varistor Vl becomes fully conductive, and in response, the voltage applied to varistor V2 When the voltage reaches 17 volts, varistor V2 becomes open. Capacitor C 1 is 3.9 nF, and capacitor c2 is 6.8 nF. According to Figure 3, The transformer core saturates after only 50 microseconds, and the voltage on the secondary side is approximately 6.2 volts. It starts to fall from 0 and settles down at about 1.8 volts. This is after 1.50 microseconds has been achieved. Balance with earth potential is perfect.

The time required for the overvoltage to be transferred from the primary to the secondary of the transformer is determined by the transformer effect. The pulse energy J=AVt (J= Energy expressed in nodules, A = current, ■ = secondary voltage, t = small waiting time Sai. As a result, smaller circuit components can be used.

PTC resistors PTC1 and PTC2 have high resistance when a short transverse pulse occurs on the line. Since most of the pulse energy cannot be switched to Ll-PTCI- It is guided back to the line through Vl-PTC2-L2, where the energy is absorbed. Ru.

During the first phase of the snow pulse, especially the first 50 or so miles until the transformer core saturates. During microseconds, energy is stored in the transformer core and capacitor. after that When Vl conducts, that is, when there is a short circuit, all the energy is returned to the line via Ll, L2.

When PTC1 and PTC2 completely switch and become high resistance, the resistor will overvoltage. absorb. The thermal energy that the resistor needs to absorb, P=U”XR, is very It's getting smaller. R is the resistance of the PTC resistor, which is now very large. .

For example, if an overvoltage such as 220V AC is applied to the line for a long time, the center point The impedance of capacitor C, S1/C, becomes very high, and all current becomes L. It will flow through L-C-L2. Here, w=2πf, where f is the frequency of the AC voltage. means wave number. Typical total current in this case is about 70 mA. Branch road Ra1 ．． V1. Ra2 is short-circuited, and the steel resistance on the negative winding side absorbs all the power as heat. will be collected. The thermal energy in this case is approximately 0.5 watts.

The nominal voltage of the varistor Vl must not be so low that it starts conducting at the supply voltage E . In other words, the nominal voltage should match the power supply voltage E.

The fact that the overcurrent and overvoltage protection circuit according to the present invention improves ground balance is the first Note that the current limiting resistors Ra and Rb of the illustrated embodiment are not found in the protection circuit of the present invention. This is clear from the fact that Ground balance for low frequency signals is determined by Ral and Ra2. However, it is also influenced to some extent by PTC1 and PTC2. center point A slightly higher frequency signal where capacitor C begins to short PTCI and PTC2. In this case, earth balance is determined by the resistance of the copper in the primary winding of the line transformer. Ru. Vl, Ral.

Ra2 no longer affects the equilibrium. In the case of high frequencies, the ground balance is Ll and L 3 and between L2 and L3, plus the primary windings Ll, L It is determined by the resistance value of copper in 2. On the other hand, in the known circuit shown in Figure 1, , resistors Ra and Rb affect the ground balance at all frequencies.

The overvoltage/overcurrent protection circuit of the present invention has the effect of improving return loss. because Since the circuit of the invention does not include the resistors Ra and Rb found in the known circuit of FIG. It is. Since no power is consumed by Ra and Rb, the impedance matching of the line can also be improved. It will be good. Both noise and harmonic distortion factors are reduced. Power to send signals to the railroad tracks can be reduced on the sending side.

The amount of attenuation and basic attenuation in mixed loss distortion can also be reduced by the overcurrent and overvoltage protection circuit of the present invention. becomes smaller. This is because the circuit of the present invention differs from the known protection circuit shown in the first factor. Since there are no Ra and Rb to be generated, the voltage in the LC circuit on the primary side of line transformer LT This is because power loss is reduced.

The protection circuit according to the invention also reduces loop attenuation compared to the known protection circuit of FIG. Ru. If possible, the balanced network on the secondary side of the line transformer LT2 or the one corresponding to Ra and Rb. This is because there is no need to As a result, the so-called attenuation of crosstalk between the transmitter and receiver becomes larger.

When an overvoltage pulse occurs, the potential of the circuit board on which the circuit of the present invention is mounted is does not change. In the configuration shown in Figure 1, the overvoltage is passed through Zener diodes Z2 and Z4. As the voltage is attenuated on the board, the potential of the board increases, creating a risk of the component catching fire. arise.

The overvoltage/overcurrent protection circuit of the present invention provides R, S, When used in a T-interface network terminal, the circuit shown in Figure 2 Voltage source E is removed. Used on the subscriber side of conventional telephone networks where voltage source E is not required. The same applies when

The embodiments of the present invention described above can be modified in many ways within the scope of the claims. be.

Summary book Exchange with two input terminals (a, b) and several output terminals (T1-T4) An overvoltage and overcurrent protection circuit for a terminal circuit is provided. Input terminal connects to two-wire line The output terminal sends analog and digital signals onto the line via a switching terminal circuit. Connected to a device for transmission. Exchange terminal circuit impedance growing transformer The primary and secondary sides of the transformer have symmetrical networks with respect to ground. be. - In the next circuit, the winding is divided into two equal parts (Ll, L2), and there is a connection between them. Capacitors (C) are connected in series. The novel feature of the present invention is the varistor (Vl) is connected in parallel with the capacitor (C). The surge pulse is When the varistor is charged to the rated voltage of the varistor, the varistor conducts and the transformer Core is #! Increase the current through the primary winding until the sum increases. Varistor on the secondary side of the transformer (v2) and Zener diodes (25 to Z8), which are transmitted to the secondary side. It helps to gradually reduce the generated surges and provide a safe voltage to the connected equipment. fulfill the eyes.

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Claims (4)

[Claims] 1. An end with two input terminals (a, b) and several output terminals (T1 to T4) an overvoltage and overcurrent protection circuit for terminal circuits or network terminals, the two input terminals The child is connected to a two-wire line, and the several output terminals are connected to the two-wire line. It is intended to be connected to equipment protected by an overvoltage and overcurrent protection circuit, and The terminal circuit transmits digital signals and analog signals bidirectionally on the line. In the overvoltage and overcurrent protection circuit, Line transformer with primary windings (L1, L2), secondary windings (L3, L4) and core (LT2), wherein the primary winding includes two winding portions (L1, L2). transformer and A capacitor (C) and two current limiting resistors are placed on the primary side of the transformer. a first earth symmetrical protection circuit comprising a capacitor (Ra1, Ra2); The sensor (C) is connected in series between the two winding portions (L1, L2) and said first terminal forming a series combination connected between two input terminals (a, b); - base symmetrical protection circuit; It is placed on the secondary side of the transformer (LT2) and controls the voltage applied to the output terminal. A second earth symmetrical protection circuit containing Zener diodes (Z5-Z8) to including, The first protection circuit includes a varistor (V1) and two temperature-dependent semiconductor resistors (P TC1, PTC2), one of the two resistors is connected to the barrier. The capacitor (C) is connected in series between one end of the star (V1) and one end of the capacitor (C); One resistor is connected between the other end of the varistor (V1) and the other end of the capacitor (C). be connected in series with The series combination body directly connects the two input terminals (a ,b) and An overvoltage and overcurrent protection circuit featuring: 2. The second protection circuit is connected in parallel between the output terminals of the secondary windings (L3, L4). 2. A second varistor (V2) connected to the second varistor (V2). Built-in overvoltage and overcurrent protection circuit. 3. The first current limiting resistor (Ra1) has a first end connected to ground and a first current limiting resistor (Ra1). 2 is connected to the one end of the first varistor (V1); The second current limiting resistor (Ra2) has a first end connected to ground and a second end connected to ground. connected to the other end of the first varistor (V1). 3. The overvoltage/overcurrent protection circuit according to claim 2. 4. A voltage source (E) for supplying current to the line in a known manner connects said two currents. connected in series between the first ends of the limiting resistors (Ra1, Ra2); and one pole of the voltage source (E) is grounded. The overvoltage/overcurrent protection circuit according to claim 3.